The present invention relates to a method of aerobic decomposition of organic solids, as well as to an apparatus for carrying out the method.
Methods are known for converting sludge from community and industrial sewage treatment plants, as well as animal excrement from places where animals are kept in large numbers, into an environmentally harmless solid by means of biological, thermal, and mechanical processing.
Such methods are based upon the known aerobic decomposition techniques (composting). The technique which has been known for a long time from the literature in the field and which is probably the one used most often requires, for example in the composting process, employing piled heaps of debris in a more or less pyramidal shape. A processing time of several months is required and little or no destruction of pathogenic organisms usually occurs.
In these known methods, an effort is made to remove considerable moisture from the decomposing material during the aerobic decomposition process.
Aerobic decomposition methods require sufficient amounts of oxygen for activation of the aerobic microorganisms.
The heaps piled up at ground level seldom reach temperatures over 50.degree. C., even at their centers. Although higher temperatures can be produced for short periods of time by turning over the heap with shovels or mechanically, thus allowing oxygen to enter the material, the decomposition process very rapidly dies down once more, because the heap is unable to take up oxygen automatically. This is primarily because the outer layers of the heap very rapidly lose their moisture as a result of weathering, dry out, and form a crust which prevents oxygen uptake (breathing of the heap).
This heap technique, which is largely dependent upon weathering and manual work (turning over), can, however, be influenced in a definitely positive fashion if the heap of debris is arranged about 30 centimeters above the ground on an air-permeable grid and protected against the weather. As the decomposition process begins, the middle of the heap warms up and the rising warm air passes through the porous bottom and allows fresh air to enter the heap. Improved decomposition efficiency is definitely achieved with this type of heap composting.
However, limits are imposed on this self-heating process, since the heat losses from the large surface of the heap are greater in this case than the heat which is liberated, so that it is impossible to reach an optimum steady temperature. This and similar findings regarding the course of the decomposition process have led to the construction of decomposition reactors.
Naturally, the reactor is limited as far as the size of the area which can accept heaps is concerned. This limitation on area, especially on the diameter of decomposition reactors, as they are known today (Schnorr-Kneer-Kahlin), results from the mechanisms which are used to empty the reactors and the mechanical limits to the stress that can be imposed upon them. The filling volume of such reactors is therefore increased by increasing the filling height. As the filling height increases and the pressure produced thereby increases as well, the number of gas-conducting pores decreases in one direction, even with very coarse pulverization of the filling material intended for decomposition, and this prevents natural air circulation and resultant input of oxygen, a condition which automatically leads to undesirable anaerobiosis of the decomposing material. The incorporation of mixers to introduce air has proven to be extremely troublesome from the mechanical standpoint and of questionable value from the chemical engineering standpoint. Likewise, forced ventilation systems using pipes incorporated into the reactor or air jets in the bottom of the reactor, even when pure oxygen is used, appear not to produce any results that justify the expense.
The systems for decomposition of sludge or animal excrement by reactors or composting drums, with addition of presorted garbage, straw, hay, sawdust or bark as so-called carbon carriers, likewise sufficiently well-known from the literature in the field, in most cases failed to live up to the expectations made for them. This is especially true for the compost produced by these installations which, as far as structure and freedom from pathogenic organisms and the like is concerned, often does not allow this product to be used safely as fertilizer on areas for agriculture.
One of the principal reasons why the well-known Kneer reactor decomposition process fails is, in addition to the delivery system, the shape of the reactor. The latter is a tall, circular cylinder, which is emptied by means of a cutting tool which runs around the bottom of the reactor, and which need not be discussed in detail here.
However, the round shape of the decomposition reactor poses a genuine problem. This is a shape which is ideal for pourable materials, but very often leads to so-called bridging when the height of the stored material increases and the stored material is damp. This bridging takes place in such a container primarily when it is to be emptied.
Since the circulating, cutting, and emptying mechanism cuts away the material and moves it to one side immediately above the bottom of the reactor and feeds it into a dump chute, the decomposed material shifts toward one side and exhibits an increased tendency to clog inside the container as a result of this settling. The resulting bridges made of decomposed material which are produced by this technique can only be broken up by vibration applied from outside or by poking the bridges with a rod from above.